As the previous poster said redundancy and also you end up having all the the traffic go up thru a single uplink to the core. So you end up potentially having a bottleneck at the 2960 uplink depending on how much traffic your users send to the core to get routed . Each switch should have at least its own dedicated link to the core switch , Ideally you could add a second 3750 to your stack and have 2 uplinks from each switch , this gives each switch its own dedicated link to the core and its own backup